The present invention relates generally to antennas for radiofrequency energy, and more particularly to antennas required to produce electromagnetic beams over wide angles of coverage volume.
U.S. Pat. No. 4,070,678 issued to Richard L. Smedes on Apr. 1, 1976 discloses a two-axis mirror antenna. This antenna has a fixed axial feed which illuminates a fixed wire grid parabola supported by a radome. The feed polarization is parallel to the grid wires of the parabola. The parabola forms the energy into a beam aimed back toward a mirror surrounding the feed. The mirror is a "half-wave plate" which rotates polarization 90.degree. and reflects the beam into space through a spherical lens which collimates the beam. This energy, being polarized orthogonal to the grid wires forming the parabola, flows through the parabola with negligible attenuation. The echo from targets reverses the procedure to be focused onto the feed. The beam is moved by tilting the mirror, giving a beam shift of approximately twice the mirror tilt angle.
The mirror antenna is a very effective device for rapid large angle beam scanning, but the hole in the mirror for the feed limits sidelobe performance and causes some loss.